PyREPL: ESC key in isearch mode has ambiguous prefix delay

Bug report

Bug description:

This issue was noted from #145398. A bare Esc press in isearch mode (Ctrl-R) does not exit search. Instead, the first Esc appears to do nothing, only after pressing Esc a second time does isearch end.

To reproduce:

1. Press Ctrl-R in PyREPL to enter isearch.
2. Press Esc once to exit isearch.

• Expected: isearch ends immediately.
• Actual: nothing happens. Press Esc again, isearch ends.

This is because BaseEventQueue.push() treats \x1b (ESC) as a prefix of longer escape sequences (arrows, function keys, etc.). When a single Esc is received, the event queue holds it and waits for the next byte to decide whether it’s a standalone Esc or the start of a longer sequence. If no further byte arrives, the Esc event is never delivered, and isearch remains active.

Apart from isearch mode, this behavior affects normal mode as well, where Esc also has a delay before it is recognized.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-154731

Start with BaseEventQueue.push() and the escape-sequence definitions in Lib/_pyrepl/unix_eventqueue.py and Lib/_pyrepl/windows_eventqueue.py. Review linked PR gh-154731 and the existing PyREPL event handling before making changes. Done means a bare Esc exits isearch immediately and is recognized without the current delay in normal mode.